1. Field of the Invention
The present invention relates generally to a radar device used in a vehicle, and more particularly to a radar device which, in detecting distances and relative speeds with respect to a plurality of vehicles in front of a vehicle mounting the radar device using an FM-CW radar, excludes roadside objects such as crash barriers (guard rails), sound-insulating walls and wire netting disposed continuously along the sides of a road.
2. Description of the Related Art
Known instruments for measuring distances between objects and their relative speeds include measuring devices using Frequency-Modulated-Continuous-Wave (FM-CW) radar. These devices transmit a modulated wave of a predetermined modulation amplitude whose frequency rises and falls periodically, receive a reflected wave reflected by an object, and detect a difference between the frequency of the transmitted wave and the frequency of the received wave as a beat spectrum; if a beat frequency in a transmitted frequency rise period (hereinafter, rise side) and a beat frequency in a transmitted frequency fall period (hereinafter, fall side) are obtained for an object, the distance to that object and its relative speed can be obtained by calculation.
However, when there are a plurality of objects, because a plurality of beat frequencies arise, it is necessary for rise side and fall side beat frequencies to be correctly associated for each object. In particular, when there is a roadside object consisting of a crash barrier (guard rail), a sound-insulating wall or wire netting disposed continuously along the side of the road, numerous peaks arise in the beat spectrum due to reflected signals from this roadside object. As a vehicle radar device able to recognize a preceding vehicle constituting an obstruction from these multiple peak spectra, there is for example the radar device disclosed in Japanese Patent Laid-Open Publication No. HEI-7-98375.
This radar device uses an FM-CW radar to detect a rise side spectrum A and a fall side spectrum B for an obstruction. In view of the fact that in the case of a stationary body the spectrum A and the spectrum B will have a frequency difference corresponding to the speed of the vehicle, the fall side spectrum is shifted in correspondence with the vehicle speed to produce a spectrum C and this is subtracted from the spectrum A to produce a spectrum D from which spectral peaks pertaining to the stationary body have been removed. This spectrum D is then subtracted from the spectrum A to obtain a spectrum E consisting only of spectral peaks pertaining to the stationary body. Objects whose spectrum intensity is not maintained continuously in the spectrum E are excluded from the obstruction and thereby the obstruction is recognized.
Another known vehicle radar device (Japanese Patent Laid-Open Publication No. HEI-7-234277) obtains peak frequencies on the basis of frequency analysis of a beat signal; obtains deviations between the peak frequencies and their adjacent peak frequencies; obtains the minimum levels between the peak frequencies; and correctly distinguishes vehicles and roadside objects by making correlations between the deviations and the minimum levels.
However, in either of these radar devices, because numerous peaks arise in the beat spectrum, as mentioned above, there are a great many combinations of rise side beat frequencies and fall side beat frequencies and consequently a large amount of processing is required to obtain the correct combinations (pairings). Also, the probability of making wrong combinations (pairings) is high.
For example, as shown in FIG. 9, a radar signal processing device may for each beam emitted perform pairing of a peak frequency detected on the rise side and a peak frequency detected on the fall side and calculate the distance to an object and its relative speed on the basis of the paired peak frequencies.
The radar signal processing device shown in FIG. 9 is made up of a signal inputting part (A/D convertor) 101, a frequency analyzing part (FFT part) 102, a peak detecting part 103, a distance and relative speed calculating part (pairing part) 104, a clustering part 105 and a targeting part 106.
The signal inputting part (A/D convertor) 101 converts a beat signal outputted from a radar unit proper (not shown) and supplies a digital beat signal to the frequency analyzing part (FFT part) 102. The frequency analyzing part (FFT part) 102 carries out frequency analysis (spectral analysis) of the digital beat signal by performing high-speed Fourier conversion processing on the digital beat signal, and outputs frequency analysis (spectral analysis) data. The peak detecting part 103 detects peaks of the beat signal on the basis of the frequency analysis (spectral analysis) data, and supplies peak frequencies to the distance and relative speed calculating part (pairing part) 104.
The distance and relative speed calculating part (pairing part) 104 performs pairing of a peak frequency detected on the rise side and a peak frequency detected on the fall side for each beam emission direction and calculates the distance to an object and its relative speed on the basis of the paired peak frequencies. When multiple peaks are detected, a distance and a relative speed are calculated for every one of multiple combinations, and a most suitable combination is determined from the calculated distance and relative speed results. The clustering part 105 groups data (distances and relative speeds) relating to the same object on the basis of the distances and relative speeds detected for each beam emission direction. The targeting part 106 calculates a center position and a bearing of the center of each object on the basis of the peak levels detected for each beam emission direction, calculates the width of the object and outputs the position (distance and bearing), relative speed and width of each object.
When due to the existence of a roadside object multiple peaks pertaining to the roadside object are detected, because the distance and relative speed calculating part (pairing part) 104 must calculate a distance and a relative speed for every one of multiple combinations and determine the most suitable combination from the calculated distance and relative speed results, this pairing processing is time-consuming. Also, because there are many different combinations, incorrect combinations may be selected. That is, wrong pairings may arise. Consequently, the time taken for the final outputting of the position, relative speed and width and so on of each object is long, and when a wrong pairing is made the detection accuracy of the positions, relative speeds and widths deteriorates.